1. Field of the Invention
This invention relates to a novel, improved process of preparing 2,6-di-tertiarybutyl-4-ercaptophenol by an electrocatalytic reduction of bis(3,5-di-tertiarybutyl-4-hydroxyphenol)polysulfide at a lead cathode in an acidic electrolyte medium. This mercaptophenol is an intermediate in the synthesis of 4,4'-isopropylidenedithio-bis-(2,6-di-tertiarybutylphenol) which has been disclosed in U.S. Pat. No. 3,576,883 as an effective pharmaceutical agent for the reduction of serum cholesterol.
In addition, the above method carried out in the presence of acetone results in a novel, improved process of making 4,4'-isopropylidenedithio-bis-(2,6-di-tertiarybutylphenol)in a single step procedure.
2. Description of the Prior Art
U.S. Pat. No. 3,479,407 teaches the preparation of a mixture of bis(3,5-di-tertiarybutyl-4-hydroxyphenol)polysulfides, comprising principally the disulfide, by a process of sulfurization of 2,6-di-tertiarybutylphenol (DTBP) with sulfur monochloride in the presence of an iodine catalyst. The polysulfides have been shown to be reduced to 2,6-di-tertiarybutyl-4-mercaptophenol by a process comprising a Zn/HCl reduction as disclosed in U.S. Pat. Nos. 3,952,064 and 3,479,407 and in Japanese Patent Application No. 73-28425. Condensation of the resulting mercaptophenol in the presence of acetone under acidic conditions results in the formation of 4,4'-isopropylidenedithio-bis-(2,6-di-tertiarybutylphenol), as described in U.S. Pat. No. 3,576,883. This reaction sequence is presented below. ##STR1##
It is generally known that organic disulfide compounds can be reduced to the corresponding mercaptans by electrocatalysis at a mercury cathode, where mercury cleaves the sulfur-sulfur bond in an initial chemical step to form the mercury mercaptide salt, followed by electrochemical reduction of the salt to the mercaptan (J. Q. Chambers, "Organic Sulfur Compounds" in "Encyclopedia of Electrochemistry of the Elements", Vol. 12, A. J. Bard, Editor, Marcel Dekker, New York, 1978, pp 393-409). There is some precedent for electrocatalytic reduction of an organic disulfide at a lead cathode. Thioglycolic acid has reportedly been obtained by electrocatalytic reduction of dithiodiglycolic acid at a lead cathode in 2N sulfuric acid (E. Larson, Ber. Dtsch. Chem. Ges., 61, 1439 (1928).